1. Field of Invention
This invention relates generally to injection molding systems and apparatus. More particularly, the invention relates to systems and apparatus specifically adapted for injection molding articles of substantially amorphous polyethylene terephthalate and similar materials.
2. Summary of the Prior Art
The use of polyethylene terephthalate (hereinafter referred to as "PET") and similar materials as the materials of choice in the formation of numerous injection molded articles is well known in the art. For example, in the bottle and container industry, the blow molding of injection molded PET preforms has gained wide acceptance, and is experiencing strong growth. Among the reasons for this is the fact that PET and similar materials offer a wide range of desirable properties. Specifically, PET materials generally evidence high strength, good clarity, and low gas permeation characteristics. Further, PET materials are comparatively easy to recycle. Accordingly, they are desirable for use in retail packaging applications.
PET and similar materials, however, present molders with significant processing problems. These problems may be at least partially explained by the fact that these materials are considered to be what is known in the art as "crystallizable" materials. By this it is meant that the randomly oriented polymer chains of the amorphous phase of the material may be caused to form a highly ordered, crystalline structure in a controllable manner. This may be accomplished either by mechanical stretching of the material so as to cause an ordered orientation of its molecules and the formation of stress induced crystals, or by controlling the temperature of the material over time in a manner which induces crystal formation and growth. More particularly, as the temperature of the material is increased from the ambient, the material passes through a number of states. Specifically, the material in its so-called "glassy" (or rigid) state at ambient temperature upon heating will sequentially pass through a glass transition temperature range, a crystallization temperature range, and a crystal melting temperature range, before it reaches its molten state.
In the glassy state, existing crystals in the material are stable, and additional crystals cannot form because the molecules are too sluggish. This is to say that the molecules of the material lack the requisite energy to move about sufficiently to induce the creation of the intermolecular bonds necessary for crystal formation. In the glass transition temperature range (which for PET is typically between about 175.degree. F. and about 185.degree. F.), the material transforms from its glassy state to a rubbery state.
In the rubbery state, crystals tend to form and grow. The rate of this crystal formation and growth is both time and temperature dependent. More particularly, the rate of crystal formation and growth follows a substantially parabolic curve on a temperature versus time graph. It, therefore, will be recognized by those skilled in the art that for PET materials the rate of crystal formation and growth typically increases with temperature from about 185.degree. F. up to about 350.degree. F., and thereafter decreases to substantially zero at about 480.degree. F. Further, the extent of crystal formation and growth depends significantly upon the length of time during which the material is permitted to reside at any given temperature within its crystallization temperature range.
The crystal melting temperature range for PET extends between about 480.degree. F. and about 490.degree. F. Above about 490.degree. F., the material exists in its molten state.
It is to be understood that the foregoing is a generalization of the crystallization properties of PET and similar materials. Variations in the properties of the particular material under consideration (such as its intrinsic viscosity, its diethylene glycol content, its water content and/or its comonomer or other additive content) may alter the melting point of the material, the crystallization behavior of the material, or both.
Furthermore, the breakdown product acetaldehyde is known to be generated in significant amounts whenever PET material is in a molten state. It is also well understood that slight changes in the melt temperature will significantly effect the rate of acetaldehyde generation. Since acetaldehyde is a potent flavorant, its presence in the melt material must be minimized during the injection molding of food or drink containers (or preforms therefor). If this is not done, detectable changes in the flavor or aroma of foods packaged in such articles (or in containers made from such preforms) may be induced. Heretofore, acetaldehyde minimization has been accomplished by maintaining the melt temperature as low as possible, while still allowing substantially clear articles (or preforms therefor) to be formed by so-called "runnerless" injection molding apparatus.
Injection molded preforms adapted for subsequent blow molding into a finally desired container form should consist of mostly amorphous material. This permits the preform to be blow molded into a desired shape easily and with a minimum of reheating. It also avoids the formation of undesireable cracks or a whitish haziness in the finished article/preform caused by the presence of excessive crystallized material therein. Further, the article/preform should have an acceptable acetaldehyde level, and be free from contaminants or defects.
In the molding of a preform in a "runnerless" injection molding process, therefore, the material temperature is invariably maintained above the minimum temperature required to maintain the material in its molten state prior to, and during, injection. Thereafter, the material is rapidly cooled in the article formation cavity of the mold to a temperature substantially below its minimum glassy phase transition temperature. This minimizes the time during which the material is at a temperature within its characteristic crystal formation temperature range, and consequently minimizes the crystal content of the finished article/preform.
A representative PET container formation process includes the steps of:
(i) injection molding a closed bottom preform; PA1 (ii) reheating the preform to the blow molding temperature (normally about 18.degree. F. to 36.degree. F. above the glass transition temperature range of the preform material); PA1 (iii) stretching the preform axially in the blow mold by means of a stretch rod; and PA1 (iv) simultaneously with the axial stretching, introducing compressed air into the preform so as to biaxially expand the preform outwardly against the walls of the blow mold so that it assumes the desired configuration.
Injection molding of preforms acceptable for later blow molding into container configurations, therefore, requires the balancing of many factors. A detailed supplement to the foregoing brief discussion of these factors is contained in the Blow Molding Handbook, by Rosato and Rosato, Hanser Publishers, New York, N.Y., 1988, and particularly chapter 14 thereof. That publication is hereby incorporated herein by reference.
In systems and apparatus for the "runnerless" injection molding of articles/preforms of the type alluded to above, a mold and a molten material transport means are commonly provided. The mold typically includes a first cavity extending inwardly from an outer surface of the mold to an inner end, an article formation cavity, and a gate connecting the first cavity to the article formation cavity. The gate defines an inlet orifice in the inner end of the first cavity, and an outlet orifice which opens into the article formation cavity.
The means for transporting the material extends from a melt source to the vicinity of the inlet orifice of the gate. These means typically include an elongated bushing residing at least partially within the first cavity. This bushing defines an elongated, axial passageway therethrough which terminates at a discharge orifice.
A "gate area", therefore, is defined by the assembled mold and bushing between the discharge orifice of the bushing and the outlet orifice of the gate. Ideally, this gate area is the portion of the system/apparatus in which the transition of the material from the molten phase present in the "runnerless" injection apparatus to the glassy phase of the completed article occurs during the time period between sequential "shots" of material.
Specifically, during the injection of a "shot" of molten material (i.e., melt), the melt flows from the discharge orifice of the bushing, through the gap between the discharge orifice of the bushing and the inlet of the gate, through the gate, and into the article formation cavity of the mold. Since the temperature of the melt is maintained above its maximum crystal melt temperature in the bushing, and the temperature of the mold is maintained well below the minimum glass transition temperature of the material, the majority of each shot cools quickly to its glassy state in the article formation cavity of mold. This results in the preform containing low crystallinity levels (i.e., an article made up of substantially amorphous PET or other similar crystallizable polymer) because the material temperature does not remain within its characteristic crystallization range for any appreciable length of time.
At the end of each "shot" however injection pressure commonly is maintained on the melt for between about 1 and 5 seconds in order to assure that the melt is appropriately packed into the article formation cavity of the mold. Thereafter, the injection pressure on the melt is released, and the article is allowed to cool in the mold for between about 10 and 20 seconds. Subsequently, the mold is opened, the article is ejected therefrom, and the mold is reclosed. The latter steps take on the order of about 10 seconds. It will be understood, therefore, that for correct system operation the temperature of the melt material must transition in the gate area of the system/apparatus during the time interval between successive material "shots" between its molten phase temperature and its glassy (rigid) phase temperature in a controlled manner.
Accordingly, thermal control of the temperature gradients in the material located in the gate area between successive "shots" of molten material is critical both to the prevention of stringing or drooling of melt material from the gate, and to the prevention of gate freeze-off. In addition, a failure to isolate the majority of the crystallized melt material formed during this transition within the vestige which extends outwardly from the completed article ejected from the mold may be detrimental not only to the efficiency of subsequent blow molding operations, but also to the quality of the final blow molded article for the reasons mentioned above.
To accomplish this thermal gate control, the art has heretofore adopted two alternative approaches. In the first of these alternatives, a mechanical melt shut off mechanism is provided by what is known as a "valve gate". In the other alternative, the axial length of the gate is increased so as to ultimately form a vestige extending outwardly from the article/preform which is substantially longer than the comparatively short vestige normally resulting from "runnerless" injection molding operations.
The valve gate utilizes a pin which is axially movable in the bushing passageway. In a first retracted position, this pin allows melt material to flow through the bushing, into the gate area, and ultimately into the article formation cavity of the mold. In a second extended position, however, the distal end of the pin closes off the gate area, and thereby shuts off the flow of melt material therethrough. Specifically, the distal portion of the valve pin either may seal the inlet of the gate, or may substantially fill the volume defined by the gate so as to accomplish melt shut off.
This mechanism has several advantages. Principle among these is the preclusion of the potentially detrimental presence of melt material in the gate area between successive "shots". The absence of melt material adjacent to the gate outlet prevents stringing of melt material between the gate and the vestige. Drooling of melt material from the gate between "shots" also is prevented for the same reason. In addition, the resulting vestige (if any) is of acceptably short length, and is composed primarily of substantially amorphous material. The latter result is achieved because the vestige is substantially thermally isolated from the melt transport means upon extension of the valve pin. Consequently, the vestige (if any) cools primarily under the influence of the surrounding gate portion of the mold which, as mentioned above, is maintained well below the minimum glass transition temperature of the material.
Apparatus of the "valve gate" type, however, requires the presence of movable elements within the mold. It, therefore, may be expected that such apparatus will be subject to maintenance and repair problems. It further may be expected that the expense involved in such maintenance, repair and related system down time will be significant. This is because the movable valve pin is located deep within the mold, and is not readily accessible.
The elongated sprue alternative, on the other hand, evolved from the fact that the portion of the mold forming the gate walls in a conventional hot runner system is inadequate for controlling the crystallization of PET and similar crystallizable polymeric materials in the gate area during the time interval between successive material "shots". More particularly, it will be understood-that the metal (typically steel) forming the gate walls in a conventional hot runner system is located between the inner end of the first cavity of the mold adjacent to the inlet orifice of the gate and the portion of the article formation cavity of the mold adjacent to the outlet orifice of the gate. In such a system, the quantity and thermal conductivity properties of the metal defining the gate are not adequate to both (1) effectively withdraw heat from adjacent melt material in the article formation cavity of the mold in a manner which assures its amorphous nature in the completed article, and (2) at the same time effectively participate in the required melt material crystallization control in the gate area.
Accordingly, the axial length of the gate in some cases has been increased by artisans in the field of this invention so as to provide a gate wall structure capable of performing both of the above functions simultaneously. This, in turn, has resulted in the presence of an elongated sprue, or vestige, projecting from the finished article or preform.
The latter alternative has the advantage that the machine/mold designer can be relatively sure that substantially all crystallized material in the completed article/preform will be contained within the sprue. The resulting article/preform, however, may be adversely effected by the presence of the elongated sprue during the blow molding operation. Specifically, cracks may form at the sprue/article interface during the blow molding operation thereby ruining the blow molded article. Further, in the event that it is elected to clip the elongated sprues from the preform prior to blow molding, either to avoid crack formation or for aesthetic reasons, significant waste material, labor and other related costs may be generated.
Finally, it is to be noted that attempts to gain thermal gate control in a conventional hot runner system by increasing the spacing between the discharge orifice of the bushing and the inlet of the gate are not attractive for material crystallization control purposes. This is because increases in gate area volume act to increase the volume in which crystallization of melt material may occur during the time interval between successive "shots". Accordingly, the quantity of crystallized material left in the gate area after the ejection of a finished article or preform tends to increase.
Such increases in the quantity of crystallized material left in the gate area between successive "shots" are significant. This is because melt flow into the gate and/or article formation cavity during each "shot" is analogous to a fountain. Specifically, the melt tends to flow through the center of the gate and/or cavity, and to progressively spill outwardly toward the gate and/or cavity walls at locations immediately beyond the material which preceded it into the gate and/or cavity. Accordingly, an increase in the quantity of crystallized material left in the gate area between successive shots of material unacceptably increases the potential for that crystallized material to be conveyed into the main body of the next article formed by the system, rather than to be maintained within the gate area or within the vestige projecting from the next article molded by the system.
Decreases in the spacing between the discharge orifice and the gate inlet orifice also are not favored. Such decreases in spacing tend to adversely effect the temperature gradients in the gate area. Thus, thermal isolation of the gate area is reduced, and control of crystal formation becomes more difficult.